1. Field
The present disclosure relates to a method and a system for controlling an image printing system in the presence of reload defects.
2. Description of Related Art
Xerographic and electrophotographic printing and marking engines schedule control patches for calibration and other machine diagnostic procedures. The control patches are printed between images in what is called “inter-document zones” (IDZ) on the photoreceptor belt and/or other image bearing surface using a calibration procedure having a desired toner area coverage, for example, as disclosed in U.S. Pat. No. 6,016,204, which is incorporated by reference herein in its entirety. The areas of the image bearing surface where toner images of the print job are printed, by contrast, are called the “customer image zones,” or “image zones.”
The control patches may include one or more toner density patches. The toner area coverage, AC is defined as the percentage of toner area covering a unit halftone cell in a sample target that is available to reflect. As known in the art, control patches may be varied uniformly for each test patch from 0 to 100%. These control patches are sensed and machine parameters may be adjusted to maintain a tone reproduction curve (TRC).
As the marking engine's performance degrades over time, different image quality (IQ) problems may arise. In particular, one IQ problem of concern is reload. Reload defects may occur when the reload performance of the toner development system degrades to the point where the toner image impacts the control patches, and/or, the control patches impacts the toner image with a reload or ghosting defect.
FIG. 1 illustrates a known toner development system 100 in an image printing system. Such a system is disclosed, for example, in U.S. Patent Application Publication No. 2006/0109487, which is incorporated by reference herein in its entirety.
The toner development system 100 includes a reservoir 164 containing developer material 166. The developer material 166 may be either of the one component or two component type; that is, it comprises carrier granules and toner particles. The reservoir 164 includes augers 168, which are rotatably-mounted in the reservoir chamber. The augers 168 serve to transport and to agitate the material within the reservoir 164 and encourage the toner particles to charge and adhere triboelectrically to the carrier granules. In one embodiment, a magnetic brush roll 170 transports developer material 166 from the reservoir to loading nips 172 and 174 of donor rolls 176 and 178. Magnetic brush rolls are well known, so the construction of roll 170 is not described in great detail. Briefly, the roll 170 includes a rotatable tubular housing within which is located a stationary magnetic cylinder having a plurality of magnetic poles impressed around its surface. The carrier granules of the developer material 166 are magnetic and, as the tubular housing of the roll 170 rotates, the granules (with toner particles adhering triboelectrically thereto) are attracted to the roll 170 and are conveyed to the donor roll loading nips 172 and 174. A metering blade 180 removes excess developer material from the magnetic brush roll 170 and ensures an even depth of coverage with developer material 166 before arrival at the first donor roll loading nip 172.
At each of the donor roll loading nips 172 and 174, toner particles are transferred from the magnetic brush roll 170 to the respective donor roll 176 and 178. The carrier granules and any toner particles that remain on the magnetic brush roll 170 are returned to the reservoir 164 as the magnetic brush continues to rotate. Transfer of toner from the magnetic brush roll 170 to the donor rolls 176 and 178 can be encouraged by, for example, the application of a suitable D.C. electrical bias to the magnetic brush and/or donor rolls. The D.C. bias (for example, approximately 70 V applied to the magnetic roll) establishes an electrostatic field between the donor rolls 176 and 178 and magnetic brush roll 170 that causes toner particles to be attracted to the donor roll from the carrier granules on the magnetic roll. The relative amounts of toner transferred from the magnetic roll 170 to the donor rolls 176 and 178 can be adjusted, for example, by applying different bias voltages to the donor rolls; by adjusting the magnetic to donor roll spacing; by adjusting the strength and shape of the magnetic field at the loading nips; and/or by adjusting the speeds of the donor rolls.
Each donor roll 176 or 178 transports the toner to a respective development zone 182 and 184 through which the image bearing surface 10 passes. At each of the development zones 182 and 184, toner is transferred from the respective donor roll 176 and 178 to the latent image on the image bearing surface 10 to form a toner image on the latter. Various methods of achieving an adequate transfer of toner from a donor roll to a latent image on an image bearing surface are known and any of those may be employed at the development zones 182 and 184.
In the toner development system 100 of FIG. 1, each of the development zones 182 and 184 is shown as having a pair of electrode wires 86 and 88 disposed in the space between each donor roll 176 and 178 and image bearing surface 10. The electrode wires may be made from thin (for example, 50 to 100 micron diameter) stainless steel wires closely spaced from the respective donor roll. The wires are self-spaced from the donor rolls by the thickness of the toner on the donor rolls and may be within the range from about 5 micron to about 20 micron (typically about 10 micron) or the thickness of the toner layer on the donor roll.
For each of the donor rolls 176 and 178, the respective electrode wires 86 and 88 extend in a direction substantially parallel to the longitudinal axis of the donor roll. An alternating electrical bias is applied to the electrode wires by an AC voltage source 190. The applied AC establishes an alternating electrostatic field between each pair of wires and the respective donor roll, which is effective in detaching toner from the surface of the donor roll and forming a toner cloud about the wires, the height of the cloud being such as not to be substantially in contact with image bearing surface 10. The magnitude of the AC voltage is in the order of 200 to 500 volts peak at frequency ranging from about 8 kHz to about 16 kHz. A DC bias supply (not shown) applied to each donor roll 176 and 178 establishes electrostatic fields between the image bearing surface 10 and donor rolls for attracting the detached toner particles from the clouds surrounding the wires to the latent image recorded on the photoconductive surface of the image bearing surface.
After development, excess toner remains on the donor rolls 176 and 178 for another trip through the donor roll loading nips 172 and 174. As successive electrostatic latent images are developed, the toner particles within the developer material 166 are depleted. A developer dispenser 105 stores a supply of toner particles, with or without carrier particles. The dispenser 105 is in communication with reservoir 164 and, as the concentration of toner particles in the developer material is decreased (or as carrier particles are removed from the reservoir as in a “trickle-through” system or in a material purge operation as discussed below), fresh material (toner and/or carrier) is furnished to the developer material 166 in the reservoir. The auger 168 in the reservoir chamber mixes the fresh material with the remaining developer material so that the resultant developer material therein is substantially uniform with the concentration of toner particles being optimized. In this way, a substantially constant amount of toner particles is in the reservoir with the toner particles having a constant charge. The developer housing or reservoir 164 may also include an outlet 195 for removing, developer material from the housing in accordance with a developer material purge operation as discussed in detail below. The outlet 195 may further include a regulator (not shown) such as an auger or roller to assist in removing material from the housing.
In one embodiment, various sensors and components within the toner development system 100 are in communication with a system controller 90, which monitors and controls the operation of the toner development system to maintain the toner development system in an optimal state. In addition to the voltage source 190, the donor rolls 176 and 178, the magnetic brush roll 170, the augers 168, the dispenser 105 and the outlet 195, the system controller 90 may, for example, communicate with a variety of sensors, including, for example, sensors to measure toner concentration, toner charge, toner humidity, bias of the magnetic brush roll, and the bias of the donor roll.
When each donor roll 176 or 178 rotates and completes a full rotation, the donor roll 176 or 178 has toner with a different charge/mass ratio than in regions where the toner has been on the roll for multiple revolutions. In particular, the developability may be less for toner in regions of the roll where toner was removed during the previous revolution. This leads to the possibility of a reload error or reload defect, which appears as a light area in the later region. As a result, a “ghost” image of a previous control patch may be printed with a toner image or vice versa.
FIG. 2 illustrates a graph of the toner mass on a region of the donor roll 176 or 178 in the toner development system 100 immediately after printing. As the donor roll 176 or 178 continues to rotate more and more toner will accumulate on the donor roll 176 or 178, thereby replenishing the toner mass on the donor roll 176 or 178. This process may take multiple revolutions of the donor roll 176 or 178. After a sufficient number of rotations, the toner mass at that region of the donor roll 176 or 178 will be completely replenished (mass Mo).
A problem arises, however, where an image to be printed requires more toner (mass M1) than that region of the donor roll 176 or 178 might be presently able to provide (mass M2), i.e., before that region of the donor roll 176 or 178 has been replenished with toner. If so, there may be the possibility of a reload artifact or defect appearing in the printed document.
One possible solution is to image control patches in an edge zone on the photoreceptor belt and or other image bearing surface to ensure that the control patches do not interfere with the customer print zone. Such a solution was disclosed, for example in U.S. patent application Ser. No. 11/931,721 filed Oct. 31, 2007, herein incorporated by reference in its entirety. However, for some print systems this may not be feasible.